Crater Rim Drive, Stop 6 - Southwest Rift Zone

About 200 years ago, as Kilauea caldera formed, large volumes of ash erupted
at the summit and lava erupted on the lower east rift zone. The ash
blankets the summit of Kilauea volcano (photograph by J.D. Griggs, U.S.
Geological Survey, August 14, 1986).

Hawaiian eruptions are characteristically very gentle. However, under
certain conditions, they can be violent. The following quote describes the
events at the summit of Kilauea during the 1790 eruption as witnessed by
Hawaiian warriors.

"The company in advance had not preceded far before the ground began to
shake and rock beneath their feet and it became impossible to stand. Soon a
dense cloud of darkness was seen to rise out of the crater, and almost at
the same instant the electrical effect upon the air was so great that the
thunder began to roar in the heavens and the lightening to flash. It
continued to ascend and spread abroad until the whole region was enveloped
and the light of day was entirely excluded. The darkness was the more
terrific, being made visible by an awful glare from streams of red and blue
light variously combined that issued from the pit below, and being lit up at
intervals by the intense flashings of lightening from above. Soon followed
an immense volume of sand and cinders which were thrown in high heaven and
came down in a destructive shower for many miles around. Some few persons
of the forward company were burned to death by the sand and cinders and
others were seriously injured. All experienced a suffocating sensation upon
the lungs and hastened on with all possible speed."

Written by Kamakau and reported in Westervelt's Hawaiian Legends of
Volcanoes.

The eruption also killed eighty Hawaiian warriors about six miles (10 km)
southwest of the summit. The warriors were returning to the Kau district to
defend it from an attack by Kamehameha. Some viewed the eruption, which
reduced the strength of a rival chiefs army, as a demonstration of Pele's
favor of Kamehameha. Kamehameha ultimately united and ruled all of the
Hawaiian Islands.

The southwest rift zone is one of two rift zones on
Kilauea volcano. The rifts are weak areas in the volcano where it is being
pulled apart. Magma, a few miles below the surface, moves from the summit
down and through the rift. It pushes sideways against the volcano, causing
it to spread. The brittle rocks above this intrusion
cannot stretch. Instead, they crack. The cracks on the southwest rift zone
are associated with a magnitude 7.9 earthquake that
occurred in 1868. Shortly after the earthquake, a batch of magma left
Kilauea's summit and traveled down the rift zone. As the magma moved down
the rift it caused the cracks we see at the surface today. Crater Rim Drive
is near the top of the photo. Photograph by J.D. Griggs, U.S. Geological
Survey, March 4, 1985.

Sometimes the magma in the rift rises to the surface to generate an
eruption. This happened twice in the early 1970s on the upper southwest
rift zone. In September of 1971, an eruption began on the floor of the
caldera and migrated into the southwest rift zone. It was the first
eruption on this rift since 1921. Although inflation of the summit of the
volcano and earthquake activity had increased in previous weeks, harmonic
tremor, an indication that magma was moving, began only
50 minutes before the eruption. The geologists living in the park were
altered by an alarm that was triggered by earthquakes. They managed to get
to the observatory in only eight minutes and witness the onset of the
eruption. The eruption lasted five days producing lava fountains, cascades
of lava into Halemaumau, flows which crossed the road, and new cracks in the
rift zone. History repeated itself in September of 1974. Again, an
eruption began on the floor of the caldera and migrated into the southwest
rift zone. The eruption lasted four days and covered much of the lava
erupted in 1971. This photograph, taken from the rim of the caldera, shows
the fissure on the caldera floor. Halemaumau is on the top, right corner of
the photo. Photograph by J.P. Lockwood, U.S. Geological Survey, September
19, 1974.

The vents for the September 1974 eruption can still be seen from the rim of
the caldera. Compare this photo to the one above.

Crater Rim Drive crosses the main fault that defines the
southwest edge of Kilauea caldera. The fault is draped by the 1790 ash
deposit, indicating that the caldera was already in the process of forming
when the ash began to erupt.